'NoneType' object has no attribute 'nominalWidthX' for empty glyph in FontBuilder

[NightFurySL2001]

Discussed in #3231

^{Originally posted by NightFurySL2001 July 30, 2023}
Is it normal for FontBuilder to throw a 'NoneType' object has no attribute 'nominalWidthX' error when calculating bounds of an empty glyph?
(E.g. passing an empty SVG to SVGPen and then use the charstring generated from it.)

The code in relation is this which is given in the FontBuilder example:

# calculat lsb
lsb = {gn: cs.calcBounds(None)[0] for gn, cs in glyphs.items()}

This raises TypeError: 'NoneType' object is not subscriptable, so when I try to isolate the issue it comes down to cs.calcBounds(None) gives 'NoneType' object has no attribute 'nominalWidthX' for cs of empty glyph.

I'm building with OpenType/CFF. Slightly related to #1558 .

---

It seems like when building a CFF table in FontBuilder, doing charString.calcBounds(None) on an empty glyph raise an error of 'NoneType' object has no attribute 'nominalWidthX'. As pointed out by @TakWolf the empty glyph is equivalent to the following command:

pen = T2CharStringPen(600, None)
pen.closePath()

or

<CharString name="uni0020">
  endchar
</CharString>

which as stated in the CFF Type 2 Charstring Format spec is a valid operation, where nominalWidthX might not be present if the glyph should use the value of defaultWidthX in CFF data.

[TakWolf]

TTF:

pen = TTGlyphPen()
pen.closePath()

This will cause:

  File "/Users/takwolf/Develop/Python/pixel-font-builder/venv/lib/python3.11/site-packages/fontTools/pens/ttGlyphPen.py", line 244, in closePath
    if self.points[startPt] == self.points[endPt]:
       ~~~~~~~~~~~^^^^^^^^^
IndexError: list index out of range

OTF:

pen = T2CharStringPen(width, None)
pen.closePath()

this will cause error on get lsb:

lsb = glyphs[glyph_name].calcBounds(None)[0]

TypeError: 'NoneType' object is not subscriptable

https://learn.microsoft.com/en-us/typography/opentype/spec/cff2charstr#42-finishing-a-charstring-outline-definition

The smallest valid CharString is simply an empty byte string.

[behdad]

The bounds are None for empty glyph indeed.

Try:

diff --git a/Lib/fontTools/fontBuilder.py b/Lib/fontTools/fontBuilder.py
index dd57a0507..9d06680af 100644
--- a/Lib/fontTools/fontBuilder.py
+++ b/Lib/fontTools/fontBuilder.py
@@ -117,6 +117,7 @@ charStrings = {
 }
 fb.setupCFF(nameStrings["psName"], {"FullName": nameStrings["psName"]}, charStrings, {})
 lsb = {gn: cs.calcBounds(None)[0] for gn, cs in charStrings.items()}
+lsb = {gn: l if l is not None else 0 for gn, l in lsb.items()}
 metrics = {}
 for gn, advanceWidth in advanceWidths.items():
     metrics[gn] = (advanceWidth, lsb[gn])

[NightFurySL2001]

I might suggest to fix it as such:

fb.setupCFF(nameStrings["psName"], {"FullName": nameStrings["psName"]}, charStrings, {})
lsb = {gn: cs.calcBounds(None)[0] if cs.calcBounds(None) is not None else 0 for gn, cs in charStrings.items()}
metrics = {}
for gn, advanceWidth in advanceWidths.items():
    metrics[gn] = (advanceWidth, lsb[gn])

or

fb.setupCFF(nameStrings["psName"], {"FullName": nameStrings["psName"]}, charStrings, {})
lsb = {}
for gn, cs in charStrings.items():
    lsb[gn] = cs.calcBounds(None)[0] if cs.calcBounds(None) is not None else 0
metrics = {}
for gn, advanceWidth in advanceWidths.items():
    metrics[gn] = (advanceWidth, lsb[gn])

since subscripting None in cs.calcBounds(None)[0] caused the error mentioned above.